Components of ScriptSig

A standard ScriptSig for a P2PKH (Pay-to-Public-Key-Hash) transaction contains two specific cryptographic items.

1. The Digital Signature

The largest component of ScriptSig is the Digital Signature.

• Algorithm: Usually ECDSA (Elliptic Curve Digital Signature Algorithm).

• Encoding: DER (Distinguished Encoding Rules).

• Purpose: To prove that the person creating the transaction knows the private key associated with the coins.

• SIGHASH Flag: The signature also includes a 1-byte suffix (usually 0x01 for SIGHASH_ALL) which defines which parts of the transaction are protected by the signature.

2. The Public Key

The second component is the Public Key.

• Format: Usually Compressed (33 bytes) or Uncompressed (65 bytes).

• Purpose: The node needs the public key to verify the signature. It also hashes this public key to ensure it matches the address (Public Key Hash) specified in the lock.

3. Raw Data Structure

In the raw hex of a transaction, these components are prefixed by their length.

• [Sig Length][Signature Bytes][Pubkey Length][Pubkey Bytes]

4. Multi-Sig ScriptSig

In more complex transactions, like Multi-Sig, the ScriptSig might contain:

• An OP_0 (due to a legacy bug in the Bitcoin source code).

• Multiple signatures from different owners.

Component	Standard Size	Purpose
Signature	~71-73 Bytes	Proof of Intent/Authorization
Public Key	33 Bytes	Proof of Identity
SIGHASH	1 Byte	Defined signing scope

In the next section, we will explore the Stack Execution Model in detail.